The invention herein described was made in the course of or under a contract or subcontract thereunder (or grant) with the Department of the Navy.
This invention relates to the weld repair of surface defects in alloys, and more particularly to such repairs conducted on cast nickel-base superalloys having a nil-ductility range.
Nickel-base alloys contain more nickel than any other element, plus alloying elements that are added to improve the mechanical and physical properties of the alloy. Nickel-base superalloys are nickel-base alloys that are strengthened by precipitation of gamma prime and/or a related phase. These materials are used in aircraft gas turbine components and other applications which require good strength, creep resistance, fracture toughness, and other mechanical properties at elevated temperatures such as 1500xc2x0 F. and higher for extended periods of time.
The selection of the types and amounts of alloying elements present in the alloy, in combination with the thermal processing, determines to a large degree the properties of the alloy. In some cases, the nickel-base superalloy is moderately strong and moderately ductile, even at high temperatures. In other cases, the combination of alloying elements causes the material to be very strong but of limited ductility even at temperatures approaching the melting point, a temperature at which many alloys become highly ductile.
In one class of nickel-base superalloys, the alloy has very small, substantially no (nil), ductility in the temperature range between the solidus (freezing) temperature of the alloy and a temperature about 600xc2x0 F. below the solidus temperature. This behavior plays a significant role in the properties of the alloy, because as the alloy is cooled through this range, or is processed or operated while in this range, the material is highly susceptible to the formation of cracks and other defects which remain in the structure upon cooling to room temperature and during service. Such cracks can also occur later in the processing, as for example in the formation of hot tears during welding and post-weld heat treatments. Examples of such nickel-base superalloys having a xe2x80x9cnil-ductility rangexe2x80x9d are Rene 108 and Mar-M246.
Articles made of nickel-base superalloys are usually cast from the melt into a mold, with investment casting being the most popular approach, and then further processed. The as-cast articles sometimes have near-surface defects such as hot tears, surface-connected porosity due to shrinkage defects, and near-surface inclusions, which are acute problems for the alloys having a nil-ductility range. These near-surface defects are deleterious to the properties of the article, either directly or by preventing the closure of interior porosity and shrinkage cavities during subsequent processing.
If the surface defects are not too severe, as is often the case, they may be repaired. However, earlier repair techniques involving welding procedures are not fully successful in removing the near-surface defects, particularly for the nickel-base superalloys with a nil-ductility range. It is often the case that the repair procedure itself leads to even further defects, which in turn must be repaired. The repair procedures are accordingly very labor intensive, time consuming, and expensive. They also result in products which may have reduced properties at the location of the repair.
An important advance in welding repair procedures was disclosed in U.S. Pat. No. 5,897,801. In this approach, the weld repair is performed with the article heated to an elevated temperature and in a controlled atmosphere. The approach works quite well for many applications. If the article is large in size, the required controlled-atmosphere heating oven is also quite large and involves a large capital expenditure. The repair cost for each article is relatively high, due to the use of the controlled-atmosphere heating oven, the long cycle time for each article, and the skill required.
There is accordingly a need for an improved technique for repairing near-surface defects in articles made of nickel-base superalloys having a nil-ductility range at elevated temperature, which achieves good results yet is less expensive than the approach of the ""801 patent. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for weld repairing a nickel-base superalloy article having a nil-ductility range. It is suitable for repairing relatively small near-surface defects, such as small surface cracks, small near-surface pits, and near-surface inclusions such as small ceramic particles. These small near-surface defects are often the cause of the rejection of castings of nil-ductility-range nickel-base superalloys. The present approach allows the repair of these smaller defects at room temperature and without generally heating the body of the article to elevated temperature, so that the repair operation may proceed much more rapidly than does the approach of the ""801 patent. The approach of the ""801 remains the preferred technique for repairing larger defects and for joining articles made of the nil-ductility-range nickel-base superalloys.
A method of weld repairing a nickel-base superalloy article having a nilductility range from a solidus temperature of the superalloy to about 600xc2x0 F. below the solidus temperature comprises the steps of furnishing the article of such a composition and having a body with a near-surface defect near a surface thereof, and providing a heat source having a heat source beam with an amperage of no greater than about 5 amperes. The diameter of the heat source beam is preferably no greater than about 0.025 inches. Examples of suitable nickel-base superalloys having a nil-ductility range include R108, Mar M-246, AF2-1DA, Udimet 500, B1900, and Udimet 700. The article may be in any form, but is typically in an as-cast state. The heat source is preferably a plasma welding torch. Examples of near-surface defects are pits, cracks, and solid inclusions such as embedded ceramic particles.
The method further includes weld repairing the near-surface defect, the step of weld repairing including the steps of locally melting the surface of the article in a region of the near-surface defect using the heat source to form a melted region, while not otherwise heating the body of the article, and thereafter allowing the melted region to solidify. The melted region preferably has a molten pool depth of no greater than about 0.030 inch, more preferably no greater than about 0.020 inch, and most preferably no greater than about 0.010 inch. The melted region is thereafter allowed to solidify.
The repair may be accomplished either by heating and melting the region of the near-surface defect without the addition of a filler metal, or by adding a filler metal into the melted region. The filler metal, where used, typically has a filler metal composition that is substantially the same as that of the nickel-base superalloy of the article.
The present approach utilizes a small, narrowly focused, low-power heat source such as a small plasma welding torch or a laser. The heat source melts only a small depth at and below the surface of the article. The portions of the article that are further from the surface are not substantially affected by the welding process. Accordingly, there is reduced concern that hot tears and strain-age cracking in a heat affected zone (HAZ) will affect the body. The present approach has the important advantage that it is accomplished at ambient temperature without heating the body of the article and does not require the use of a special atmosphere. The processing is therefore much faster and uses less capital equipment than required for the approach of the ""801 patent, although the present approach is more limited in the types of defects that may be repaired than is the approach of the ""801 patent. For example, the present approach is not concerned with blade-tip and other larger-size weld repairs.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.